Prior art coating apparatuses include a box-like container and a diffuser plate dividing the container into a coating chamber and a plenum. Very fine pores perforate the diffuser plate such that a gas, typically air, is forced through the plate from the plenum to suspend a powder in the coating chamber. The powder suspended in this manner is sometimes referred to as a “fluidized bed”. The upper end of the container is open to provide for immersion of a heated article in the fluidized bed. The powder melts upon contact with the heated article to form a coating.
Coating problems can result from, or be aggravated by lack of relative movement between the article being immersed and the fluidized bed of material.
It is known to include pneumatic or electric agitation devices secured to the container of prior art coating apparatuses. The agitation devices promote particle movement within a fluidized bed of material in the coating chamber. The resulting agitation, however, is uneven with the particles nearer the devices being moved to a greater extent. Agitation of particles by such mechanical means is of particularly limited benefit in those situations where the fluidized bed includes a large mass of material.
The sources for coating problems in powder coating apparatuses of the prior art was not limited to lack of motion of the part being dipped. Coating problems would also result from, or be aggravated by, particle size gradients and temperature gradients. A powdered coating material includes particles of varying diameter. The particles can become stratified in the fluidized powder over time because of gravity effects, with larger particles concentrating near the bottom of the coating chamber. The amount of heat that is required to raise the temperature of the powder by a given amount will increase as the average particle size increases. As a result, a temperature gradient develops in the fluidized bed of prior art apparatuses creating a relatively “cold” stratum near the bottom of the coating chamber.
The formation of a temperature gradient in the fluidized bed of the prior art devices results in the application of coatings of non-uniform thickness. Insufficient heat for melting the larger and cooler particles clustered near the bottom of the coating chamber results in the application of thin coatings susceptible to pinholes. The smaller and hotter particles near the upper surface of the fluidized bed sometimes results in excess powder melting and undesirable formations such as “icicles” or “angel hair”.
Continuous coating lines are particularly prone to developing a temperature gradient in the fluid bed. The fluid bed in this type of coating process is typically situate in close proximity to the preheat and post fuse ovens. The resin in the top stratum of the fluid bed approaches its melt point because sufficient heat is transferred from the continuous dipping of parts and from the ovens. It is necessary to refrigerate the fluidizing air to prevent the resin from melting.
The relatively cool fluidizing gas increases in temperature as it travels upwardly through the fluidized bed, resulting in the removal of heat from the powder in a disproportionate manner, and an increase in the temperature gradient. The increased temperature gradient further increases the likelihood for coating thickness variation or other undesirable formations.